Faber–Jackson relation

of elliptical galaxy, and was presented by the astronomers Sandra M. Faber and Robert Earl Jackson in 1976.

In 1962, Rudolph Minkowski had discovered and wrote that a "correlation between velocity dispersion and [luminosity] exists, but it is poor" and that "it seems important to extend the observations to more objects, especially at low and medium absolute magnitudes".

depends on the range of galaxy luminosities that is fitted, with a value of 2 for low-luminosity elliptical galaxies discovered by a team led by Roger Davies,[2] and a value of 5 reported by Paul L. Schechter for luminous elliptical galaxies.

[3] The Faber–Jackson relation is understood as a projection of the fundamental plane of elliptical galaxies.

One of its main uses is as a tool for determining distances to external galaxies.

The gravitational potential of a mass distribution of radius

and assume this is a constant (which from a fundamental theoretical point of view, is a totally unjustified assumption) to get Using this and combining it with the relation between

, this results in and by rewriting the above expression, we finally obtain the relation between luminosity and velocity dispersion: that is Given that massive galaxies originate from homologous merging, and the fainter ones from dissipation, the assumption of constant surface brightness can no longer be supported.

Empirically, surface brightness exhibits a peak at about

Even first-ranked cluster galaxies do not have constant surface brightness.

A claim supporting constant surface brightness was presented by astronomer Allan R. Sandage in 1972 based on three logical arguments and his own empirical data.

In 1975, Donald Gudehus showed that each of the logical arguments was incorrect and that first-ranked cluster galaxies exhibited a standard deviation of about half a magnitude.

In the case of elliptical galaxies, if one can measure the central stellar velocity dispersion, which can be done relatively easily by using spectroscopy to measure the Doppler shift of light emitted by the stars, then one can obtain an estimate of the true luminosity of the galaxy via the Faber–Jackson relation.

This can be compared to the apparent magnitude of the galaxy, which provides an estimate of the distance modulus and, hence, the distance to the galaxy.

By combining a galaxy's central velocity dispersion with measurements of its central surface brightness and radius parameter, it is possible to improve the estimate of the galaxy's distance even more.

This standard yardstick, or "reduced galaxian radius-parameter",

, devised by Gudehus in 1991, can yield distances, free of systematic bias, accurate to about 31%.